There are basically two types of ice makers: household units in refrigerators; and self-contained commercial units for use in hotels, restaurants, bars, hospitals and other establishments that require large amounts of ice. Commercial units are further dividable into two types, depending on the type of ice they make: flaked or cubed.
Unlike household ice makers which freeze water in a tray with cool air in a refrigerated compartment, a commercial ice cube maker circulates a steady stream of water over a chilled ice mold to deposit thin layers of ice in the pockets of the mold for building into ice cubes. Water that does not freeze after being circulated over the ice mold is collected in a sump and recirculated over the chilled mold until it cools enough to freeze. After ice cubes are formed, they are harvested from the mold and stored in an unrefrigerated ice bin from which they may be retrieved. The bin remains unrefrigerated so that the ice melts slowly, thereby preventing it from sticking together.
Cold refrigerant from a refrigeration circuit chills the ice mold. In a typical refrigeration circuit, a compressor driven by an electric motor that compresses refrigerant to a high pressure and supplies it to a condenser. The condenser cools the compressed refrigerant with air blown across coils with a fan or with water. The refrigerant is then passed through an expansion valve, the expansion valve dropping the pressure of the refrigerant considerably, thereby cooling it. The cooled refrigerant then flows through copper tubing that has been welded to the back of a copper plate, called the evaporator plate. Welded to the evaporator plate is a lattice-like copper structure that is used to mold the ice into cubes. Together, the lattice-like structure and the evaporator plate form the ice mold. Taken together, the ice mold and the copper tubing are simply referred to as the evaporator.
An electronic controller, sometimes microprocessor-based, operates the fans, motors, pumps and valves that control the functioning of the ice maker.
Commercial ice makers are expected to continuously and reliably produce substantial amounts of ice. They are used in service industries, where a unit breaking down or producing insufficient ice causes disruptions of service. When there is no ice, service suffers and customers are quickly irritated: few people, for example, enjoy warm soft drinks. An unreliable ice maker will quickly erode a firm's goodwill and its business. An unreliable ice maker also costs the manufacturer money and goodwill. When the ice maker is down, its manufacturer must spend money either quickly repairing it or furnishing substitute ice.
A better ice cube is generally not sought, just a less expensive one, ice being a fungible commodity. Therefore, in addition to reliability, holding down the cost of an ice maker by controlling the cost of manufacturing and operation is a paramount concern in the art. Low cost operation requires that ice be made efficiently by conserving electricity and water; and further that the ice maker be nearly maintenance-free, as down-time for maintenance costs money and someone must be paid to do it. Low cost operation and maintenance must extend over many years, as ice makers are expected to have long, productive lives.
Efforts to achieve low cost, efficient, highly reliable operation are beset by a number of problems, most of all by the fact that cost, efficiency and reliability are frequently traded one for the other in designing and manufacturing ice makers. Some, but by no means all, of the common problem areas are: manufacturing a structure for ice making operation; harvesting ice; handling of water; manufacturing the evaporator; and generally controlling the operation of the ice maker, including initiating and terminating freezing and harvesting, purging and detection of ice levels in the ice bin.
Problems associated with harvesting the ice center around the fact that ice cubes freeze to the surfaces of the ice molds. The most common harvesting method is, not surprisingly, to unfreeze them by quickly warming the evaporator and melting the ice immediately adjacent to the surfaces of the mold. To warm the evaporator, the cycle of the refrigeration circuit is essentially reversed by opening a solenoid-operated valve (termed a hot gas solenoid or valve) to permit hot refrigerant from the compressor to flow directly into the evaporator. This method is termed in the art a hot gas defrost.
Despite the unfreezing, the cubes often do not simply fall out of the ice mold. Water from the melting ice creates a "capillary"-like action that tends to suck the cubes into the pockets of the ice mold. Gravity is often used to overcome this capillary-like action. The evaporator is oriented so that the pockets of the ice mold face down, or it is placed vertically and equipped with downwardly slanting pockets. However, even gravity cannot always be relied on to ensure that all the ice cubes are harvested simultaneously for quick harvesting and energy efficiency. Mechanical means are sometimes used in the place of, and sometimes in conjunction with, gravity to nudge or assist the ice. To simplify the mechanical means, water is recirculated over the ice mold until ice bridges are formed between the ice cubes thereby connecting the cubes into a single sheet of ice that can be pushed out of the mold. The bridges are thin and usually break easily after harvesting. Using a mechanical means for dislodging ice, however, increases the cost of manufacturing and makes the ice maker more prone to malfunction. Further, in order to freeze ice bridges between ice cubes, the freezing or icing portion of an ice making cycle must be extended to ensure that sufficiently strong ice bridges are formed between all the cubes in the pockets. Increasing the freezing time reduces ice making capacity and efficiency.
The problems of water are how to keep it from leaking out, and how to reduce its corrosive effects on equipment. Making ice requires a lot of water, and therefore also requires a water tight means of handling it so that it will not spill on the floor, get electrical components wet or corrode the interior of the ice maker. When orienting an evaporator vertically, water to be frozen cascades down the front of the ice mold, causing water to splash and creates a waterfall of unfrozen water at the bottom of the evaporator. The unfrozen water is collected in a reservoir or sump and recirculated over the evaporator. Constructing a structure to deal with this water without leaking usually involves seals having all sorts of clamps, screws, and other types of fasteners to make them water-tight. Consequently, assembly, maintenance and repair are complicated; the number of possible failure modes increases; and costs generally go up. Protecting metal parts against corrosion caused by the water and humidity, or using corrosion-resistant metals in the parts, also costs money and assembly time.
In addition to designing an evaporator that improves harvesting, manufacturing them tends to be expensive. In an evaporator refrigerant passes through a coiled copper tube. Copper is chosen because of its inherent property of good heat transference. The copper tube is welded to an evaporator plate in a coiled fashion. A lattice-like copper structure is then welded to the other side of the evaporator plate for creating the ice mold. Welding ensures good transference of heat. The entire evaporator is constructed of copper, as mating copper against other types of metals generally reduces rates of heat transfer. Constructing the evaporator is, consequently, labor intensive and expensive. Further, only one side of an evaporator can be used to make ice; a second plate cannot be easily welded to the copper tube once the first has been welded.
Finally, the problems of controlling the operational cycle of the ice maker--ice-making and harvesting of the ice particularly--are numerous.
One of the biggest problems is determining when to initiate harvesting. As the refrigeration circuit transfers heat from water that will be made into ice to air (in air cooled systems) or to cooling water (in water cooled systems), the ambient temperature of the air and the temperature of the water supplied to the ice maker directly effects the amount of time that is required to freeze the ice. Customers expect and want an ice maker to function in uncontrolled climates, such as outdoors. An ice maker is thus often subjected to temperature extremes of air and water. Consequently, since the refrigeration capacity of the ice maker is fixed, the amount of time that it takes a particular ice maker to freeze the water into ice cubes and to initiate the harvesting cycle changes considerably during the course of the year when out-of-doors, or possibly when it is moved between locations.
The freezing portion of the ice making cycle should continue, for energy efficiency and to achieve maximum ice making capacity, only as long as is necessary to ensure that, for a given air and water temperature, the proper freezing of the ice and its prompt harvesting. One approach to determining when to begin harvesting is by monitoring the actual ice build-up on the evaporator with a mechanical probe. However, mechanical probes are not always reliable, as they malfunction and must be properly adjusted to function properly and efficiently. They also complicate the ice making apparatus, increasing manufacturing costs and maintenance problems. Many ice makers, therefore, trade efficiency for simplicity and reliability: they use timers to initiate harvesting, the time being set long enough to ensure proper freezing of the ice cubes over a predefined range of ambient air and water temperatures that the ice maker is designed to face.
Similarly, heating of the evaporator should only last as long as is necessary to complete harvesting. Heating melts ice. Where the capacity of the evaporator is low, a significant fraction of the pounds of ice may be melted unless harvest is carefully controlled. The result of an unnecessarily long harvest, in addition to a lot of water, is a warm evaporator that takes longer and more energy to chill and a longer operational cycle that reduces capacity.
A control system of an ice maker, again for reasons of efficiency and reliability, must further decide when to stop making unneeded ice and when to resume making ice. The ice bin must therefore be equipped with a reliable ice level detection system.